There are a number of reasons for precisely determining the temperature or temperature change of a substance, such as a liquid, in a storage tank. Liquid in a tank expands or contracts with temperature changes. An example of a reason why accurate temperature change measurements are desired involves the field of tank testing for leakage. In order to determine whether the tank leaks, it is necessary to compensate for liquid volume variations caused by temperature changes over the period of time during which the tank is being tested.
Leakage of even a small amount of liquid from an underground storage tank such as may be used to store fuel such as gasoline, is unacceptable. A number of methods and systems have been proposed for detecting leaks in such storage tanks. In order to gather data for temperature compensation calculations, some prior art devices employ a single temperature sensing device such as a thermistor in an electrical circuit. The assumption may be made that a single sensor is adequate. Alternatively, one such single sensor apparatus employs a fluid circulating system for purposes of keeping the temperature relatively the same at all locations within the tank. Thus if the temperature begins to rise, no matter where the sensor is located, it will tend to determine relatively accurately the change in temperature throughout the tank. An example of this system is shown in U.S. Pat. No. 3,580,055. Another system employing a single thermistor for measuring temperature in a leak detecting system is shown in U.S. Pat. No. 4,186,591.
Examples of temperature measuring systems employing a plurality of thermistors at predetermined locations in the tank are shown in U.S. Pat. Nos. 4,362,403 and 4,386,525. In these patents, each temperature sensor is individually coupled to the meter to indicate the liquid temperature at that particular location in the tank. The thermistors are located at the vertical centers of equal volumes of the liquid in the tank. Once each of the individual sensors is interrogated and readings are recorded, the average temperature of the liquid in the tank can be determined. For seven sensors, seven separate measurements and calculations are necessary to determine average temperature.
It has thus been recognized that temperature can vary with level in the liquid in a storage tank. It has also been recognized that it is desirable to determine a temperature that is representative of the entire tank. The '403 patent teaches that it is useful to measure the temperature at several levels within the tank in order to determine average temperature.
Another approach for measuring average temperature of fluid in a storage tank is shown in U.S. Pat. No. 2,753,713. A plurality of different length resistance wires of equal resistance values are immersed in the liquid. The longest completely immersed wire is interrogated to determine its total resistance, which is related to the temperature of the liquid in which it resides. This, of course, gives equal weight to each molecule of liquid in thermal contact with the wire, not accounting for normally occurring temperature stratification of the liquid in the tank.